1. Field of the Invention
The present invention relates generally to electrodes for use in electrically stimulating medical devices, and more particularly, to extra-cochlear electrodes for use in stimulating hearing prostheses.
2. Related Art
Stimulating hearing protheses such as cochlear implants, vestibular array stimulation devices and auditory brain implants (ABIs) have been widely employed to assist those with sensorineural hearing loss. For example, in a cochlear implant, auditory sensations arise when a flow of electrical current between electrodes stimulates auditory neural tissue in the cochlea. The use of one or more reference electrodes implanted external to the cochlea, known as extra-cochlear electrodes (ECEs) allows alternative stimulation modes to be employed. In one conventional form, the ECE extends on its own lead, and is implanted external to the cochlea between the temporalis muscle and skull or other suitable anatomical location.
Two main stimulation modes are commonly used for stimulation. In bipolar mode, the selected active electrode and reference electrode are both positioned within the cochlea, typically close together. The use of an ECE allows for monopolar stimulation, in which current flows between an electrode within the cochlea, and an ECE. Monopolar stimulation generally produces auditory percepts using much lower current levels than is possible with bipolar modes of stimulation.
Existing ECEs are formed as so-called hardball electrodes, in which the tip of the electrode is formed as a small platinum ball. These electrodes, however, are not readily removable in the event that explanation is required. In particular, when a device is implanted into a tissue (such as when a reference electrode of a cochlear implant, is implanted under the temporalis muscle) the body's immune response causes tissue to encapsulate the device resulting in the device being mechanically locked, or keyed, into place. When it comes time for the device to be explanted, this encapsulation must be overcome, for example broken, so that the device can be removed. For a reference electrode, the force required to extract the electrode should be less than strength of the lead and/or electrode, or the lead and/or electrode may break during removal.